1. Field of the Invention
The invention relates to detecting the audible sounds and sub-audible sounds (infra-sound) made by the heart, pulse and circulatory system, particularly with respect to non-invasive cardiological sensors. Generally, the invention relates to non-invasive detection of body sounds for the purpose of diagnosis such as by spectral analysis.
2. Description of the Prior Art
Phonocardio sensors of present day design are inadequate and even hazardous for numerous applications. Appropriate sensors are desirable in magnetic resonance imaging (MRI) systems for sensing cardiovascular sounds such as pulse as well as breathing rate and temperature. In MRI systems, gated imaging is desirable in reducing image deterioration resulting from blood and fluid pressure waves related to pulse. Such waves generate tissue motion and blur MRI images which may be time exposed for as long as four minutes. There are not any known phonocardio sensors capable of safely sensing the pulses of all patients.
Electrically based sensors, which have metallic components such as electrical conductors and wires, generally present a shock hazard, and in MRI applications, a burn as well as a shock hazard. In MRI systems, radio frequency currents induced in metallic components presents a shock hazard to the patients monitored by such sensors and radio frequency heating induced in such metallic components presents a burn hazard. Additionally, lightning striking a hospital or even near a hospital can generate large induced currents in wires attached to patients. Less than a milliampere of current can result in cardiac arrest. In addition, such electrical medical sensors utilize very high input impedance amplifiers to mitigate the shock hazard. High input impedance amplifiers are, however, sensitive to picking up electrical noise and interference thereby diminishing the quality of the data provided and reducing the usefulness of such devices.
One form of phonocardio sensor in use today is the electrical microphone. Such phonocardio microphones suffer from the shock and burn hazards described above. Additionally, such microphones suffer from severe one-over-frequency (1/f) or "pink" noise, which propagates through the detectors and amplifiers, generally rendering such devices useless below approximately 25 Hz and requiring that the sensors be utilized in very quiet environments. It is appreciated that MRI systems are acoustically noisy and utilize the intense high frequency magnetic fields which induce the hazardous radio frequency currents and heating. The records generated by such phonocardio microphones are generally of poor quality.
In a small number of advanced centers for MRI research, a crude electrical sensor is utilized which also suffers from the shock and burn hazards described above. This sensor utilizes a light source and a detector to monitor flushing of the skin. When skin is pressed or stretched, cycles of red flushing is detectable at the pulse rate in many patients. Although this flushing affect is barely detectable, under appropriate conditions of illumination, a sensor of this type is operative in approximately 80% of the population. The remaining 20% cannot be so monitored.
Sedated patients that were monitored utilizing electrical wires have been severely burned. Patients have even died because they were not phonocardiologically monitored because of the hazard of burns.
As discussed above, phonocardio electrical microphones suffer from severe pink noise rendering such devices useless below approximately 25 Hz. It is believed that most of the significant body sounds occur below 25 Hz. Utilization of the prior art sensors renders this spectral region unavailable to physicians and medical researchers. The lack of an adequate low frequency sensor has prevented medical research into low frequency spectral analysis.
Thus, it is appreciated that in MRI systems, non-metallic sensors are preferred although heretofore unavailable. Additionally, non-invasive sensors are always preferred over invasive sensors for equivalent information for the shock and burn hazard reduction that such sensors provide.